For establishing the cleanliness of flowing waters, or in wastewater- and clarification plants, the liquids in TOC (total organic carbon) samples, in which also volatile organic carbons (VOC volatile organic carbon) are contained, are examined for organic carbon content or for chemical oxygen demand (COD). The COD-value is an important parameter for classifying the degree of fouling of river water and in wastewater- and clarification plants. The fundamental principle of most methods for determining chemical oxygen demand involves treating a sample with a known excess of an oxidizing agent and then ascertaining consumption of oxidizing agent. The amount of consumed oxidizing agent is converted into the equivalent amount of oxygen.
Another method for digesting TOC-samples involves the application of UV-radiation.
Contained in the reaction mixture, however, are chloride ions, which disturb the ascertaining of the chemical oxygen demand, because they likewise consume oxidizing agent and so indicate either a too high chemical oxygen demand or mean that insufficient oxidizing agent is present for converting all TOC into carbon dioxide CO2. For suppressing this effect, it is known to add to the reaction mixture mercury salts for masking the chloride ions in the liquid sample. Most often, mercury sulfate is applied. This salt has the greatest effectiveness, when it is present in a 10-times excess in comparison to the chloride content. The application of a so strong poison, as characterizes mercury sulfate in this concentration, however, burdens and endangers the operating personnel and the environment. The reaction mixture cannot be fed directly back into the water system, but, instead, must be treated and disposed of complicatedly and at high cost.
Known from DE 29 32 444 A1 is a method for separating disturbing components from a liquid, in the case of which a liquid sample is acidified with a difficultly volatile acid, wherein the chloride ions react with the acid to form hydrochloric acid gas. Then, the arising hydrogen chloride gas is purged from the liquid sample using a carrier gas. In such case, the vessel containing the liquid sample is heated. The sample-acid mixture is heated to a temperature between 30° C. and 60° C. The carrier gas is fed into the heated reactor containing the liquid sample and the acids and then brought through a removal system, where the hydrochloric acid gas deposits on the inner surface of the removal system, which is cooler than the reactor, and is absorbed by condensed water from the liquid sample. The carrier gas is fed via a pump system back to the sample-acid mixture in the reactor. By heating the sample-acid mixture, the vapor pressure of the VOC fractions in the sample is increased, so that the VOC is driven out of the sample with the carrier gas and is no longer available for the analysis.